Examples of phosphor coating methods for gas discharge lamps are described in U.S. Pat. Nos. 3,551,180, 2,987,414, 4,340,512, and in European Patent Application EP 0 479 300 A1. Generally, the phosphor particles contained in the coating layer do not adhere well by themselves to the glass envelopes used in gas discharge lamps without the aid of a suitable binding agent. In fluorescent lamps, the binding agent consists of a polymeric material, such as polyethelyene oxide, and a finely divided, high surface area, aluminum oxide. A generally preferred aluminum oxide binder is Aluminum Oxide C (available from Degussa) which is a gamma aluminum oxide, .gamma.-Al.sub.2 O.sub.3, having a particle size of about 20 nm. An aqueous coating suspension containing these binding agents and an ultraviolet (UV) stimuable phosphor is applied to the interior surface of the glass envelope and dried to form a phosphor coating layer. The polymeric binder is removed from the coating layer during a subsequent high temperature lamp baking operation. The aluminum oxide binder is not removed during the subsequent processing steps and remains in the phosphor coating layer of the finished lamp.
The presence of aluminum oxide in the phosphor layer does not pose a significant problem for fluorescent lamps. For example, .gamma.-Al.sub.2 O.sub.3 is nearly transparent to the 254 nm resonance radiation generated by the mercury discharge. (The optical gap or .alpha.-Al.sub.2 O.sub.3 is in the vicinity of 200 nm and shifts to longer wavelengths, &gt;185 nm, for .gamma.-Al.sub.2 O.sub.3.) Thus, the aluminum oxide improves the adherence of the coating layer without absorbing the UV radiation used to excite the phosphor. This situation changes however for other types of gas discharge lamps which utilize UV radiation occurring in the vacuum ultraviolet (VUV), region, less than about 170 nm (e.g., Xe excimer and neon gas discharge lamps). In those lamps, the aluminum oxide in the phosphor coating absorbs VUV radiation emitted from the gas discharge. Unlike the phosphor in the coating, the VUV radiation absorbed by the .gamma.-Al.sub.2 O.sub.3 is not converted to visible radiation but is instead dissipated through non-radiative loss processes associated with the bulk material. Hence, in VUV applications, the presence of aluminum oxide in the phosphor coating causes a reduction in lamp efficacy. If the aluminum oxide binder is removed from the phosphor coating, the lamp efficacy increases but the coating easily falls off the lamp envelope. Thus, it would be an advantage to have a binding agent which provides adherence characteristics similar to finely dividied, high surface area, aluminum oxide without causing a significant reduction in lamp efficacy.